The present invention is directed to a process for preparing a fibrillated article, the process comprising the steps of fibrillating an oriented, high melt strength polypropylene foam. The foam may be prepared by extruding a foamable mixture comprising a high melt-strength polypropylene and a blowing agent, and orienting in at least one direction.
The present invention is further directed to a process for producing a fibrillated article, prepared from an oriented foamed polymer. The fibrillated articles are useful as tape backings, filters, thermal and acoustical insulation and as a diffuse reflector for use in optical applications such as computer displays and as reinforcement fibers for polymers or cast building materials such as concrete.
In another aspect, the present invention provides a process for preparing a fibrillated article, using a foamable mixture comprising a major amount of a high melt-strength polypropylene and a minor amount of a second polymer component comprising a semicrystalline or amorphous thermoplastic polymer. Polymer mixtures comprising a high melt-strength polypropylene and two or more added polymers are also within the scope of the invention.
In another aspect, the present invention provides fibrillated articles prepared by the process of the invention. The present invention also provides fibrillated articles that comprise an oriented, high melt strength polypropylene foam having a fibrillated surface. The fibrillated surface may be fibrous or schistose in nature. The fibrillated article are useful as tape backings, filters, thermal and acoustical insulation and as reinforcement fibers for polymers or cast building materials such as concrete.
In another aspect the present invention provides polymeric microfibers prepared by the process of the invention. The microfibers are generally between about 0.5 and 10 xcexcm in cross section, have relatively uniform fiber diameters, are low cost and may be prepared from readily available starting materials. The process of the invention can produce many different profiles of fibrillated article by fibrillation of the corresponding foam profile, such as sheet, blocks, tubes, cylinders, or rods.
As used in this invention:
xe2x80x9cAlpha-transition temperaturexe2x80x9d,Txcex1c, is the temperature at which crystallite subunits of a polymer are capable of being moved within the larger lamellar crystal unit. Above this temperature lamellar slip can occur, and extended chain crystals form, with the effect that the degree of crystallinity is increased as amorphous regions of the polymer are drawn into the lamellar crystal structure.
xe2x80x9csmall-cell foamxe2x80x9d means a foam having cell sizes of less than 100 micrometers (xcexcm), preferably 5 to 50 xcexcm;
xe2x80x9cclosed-cell foamxe2x80x9d means a foam that contains substantially no connected cell pathways that extend from one outer surface through the material to another outer surface;
xe2x80x9coperating temperaturexe2x80x9d means the temperature that must be achieved in the extrusion process to melt all of the polymeric materials in the melt mix;
xe2x80x9cexit temperaturexe2x80x9d and xe2x80x9cexit pressurexe2x80x9d mean the temperature and pressure of the extrudate in the final zone or zones of the extruder and preferably in the die;
xe2x80x9cmelt solution xe2x80x9d or xe2x80x9cmelt mixturexe2x80x9d or xe2x80x9cmelt mixxe2x80x9d means a melt-blended mixture of polymeric material(s), any desired additives, and blowing agent(s) wherein the mixture is sufficiently fluid to be processed through an extruder;
xe2x80x9cneat polymerxe2x80x9d means a polymer that contains small amounts of typical heat-stabilizing additives, but contains no fillers, pigments or other colorants, blowing agents, slip agents, anti-blocking agents, lubricants, plasticizers, processing aids, antistatic agents, ultraviolet-light stabilizing agents, or other property modifiers;
xe2x80x9cfoam densityxe2x80x9d means the weight of a given volume of foam;
xe2x80x9cdensity reductionxe2x80x9d refers to a way of measuring the void volume of a foam based on the following formula:       ρ    R    =            [              1        -                              ρ            f                                ρ            o                              ]        xc3x97    100    ⁢    %  
where xcfx81R is the density reduction, xcfx81f is the foam density, and xcfx81o is the density of the original material;
xe2x80x9cpolydispersityxe2x80x9d means the weight average cell diameter divided by the number average cell diameter for a particular foam sample; it is a means of measuring the uniformity of cell sizes in the sample;
xe2x80x9cuniformxe2x80x9d means that the cell size distribution has a polydispersity of 1.0 to 2.0;
xe2x80x9csphericalxe2x80x9d means generally rounded; it may include spherical, oval, or circular structure;
xe2x80x9cfibrillosexe2x80x9d or xe2x80x9cfibrousxe2x80x9d means having elongated filament-like or thread-like structures;
xe2x80x9cschistosexe2x80x9d means having parallel plate-like ribbons or flakes;
xe2x80x9cpolymer matrixxe2x80x9d means the polymeric, or xe2x80x9cnon-cell,xe2x80x9d areas of a foam;
xe2x80x9cxcex1-olefinxe2x80x9d means an olefin having three or more carbon atoms and having a xe2x80x94CHxe2x95x90CH2 group.